U.S. patent number 4,378,470 [Application Number 06/247,229] was granted by the patent office on 1983-03-29 for interface circuits for connection to non-dedicated telephone lines.
This patent grant is currently assigned to Northern Telecom Limited. Invention is credited to Stephen D. Alvey, Tapio H. Murto.
United States Patent |
4,378,470 |
Murto , et al. |
March 29, 1983 |
Interface circuits for connection to non-dedicated telephone
lines
Abstract
Receive and transmit paths, for example for frequency shift
keyed signals, are coupled to an interface circuit including a line
transformer which is permanently connected to the telephone line
via a high impedance. The effect of this high impedance on received
signals is counteracted by an amplifier and/or a tuned circuit
comprising a winding of the transformer. Switches in the interface
circuit, responsive to received signals, serve to by-pass the high
impedance, and reduce the gain of the amplifier and/or eliminate
the effect of the tuned circuit, to permit normal coupling of
signals between the telephone line and the receive and transmit
paths.
Inventors: |
Murto; Tapio H. (Nepean,
CA), Alvey; Stephen D. (Kanata, CA) |
Assignee: |
Northern Telecom Limited
(Montreal, CA)
|
Family
ID: |
22934118 |
Appl.
No.: |
06/247,229 |
Filed: |
March 25, 1981 |
Current U.S.
Class: |
379/93.05;
379/403; 379/93.28 |
Current CPC
Class: |
H04M
11/066 (20130101) |
Current International
Class: |
H04M
11/06 (20060101); H04M 011/00 () |
Field of
Search: |
;179/1C,2C,2DP,2A,2AM,1MN,6.07,6.11,175.2C |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IBM Technical Disclosure Bulletin, vol. 17, No. 2, Jul. 1974, pp.
477-478, Chrisfield, R. P., Wrage, R. H. "Time Sharing Circuit for
Voice and FSK Data"..
|
Primary Examiner: Rubinson; G. Z.
Assistant Examiner: Brady; W. J.
Attorney, Agent or Firm: Haley; R. John
Claims
What is claimed is:
1. An interface circuit for connection to a non-dedicated telephone
line, said circuit comprising:
switching means having a first state and a second state;
means for presenting a high impedance to the telephone line in the
first state of the switching means, whereby normal use of the
telephone line is undisturbed;
means for coupling signals from a transmit path to the telephone
line and from the telephone line to a receive path in the second
state of the switching means; and
means for counteracting the effect of said high impedance on
signals received via the telephone line in the first state of the
switching means, whereby said signals are coupled to the receive
path.
2. An interface circuit as claimed in claim 1 wherein the means for
coupling signals comprises a transformer and the means for
presenting a high impedance comprises a high impedance via which a
winding of the transformer is coupled to the telephone line in the
first state of the switching means, the high impedance being
by-passed in the second state of the switching means.
3. An interface circuit as claimed in claim 1 wherein the means for
counteracting the effect of said high impedance comprises a tuned
circuit for counteracting the effect of said high impedance on
signals received via the telephone line in the first state of the
switching means and having frequencies at about the resonance
frequency of the tuned circuit.
4. An interface circuit as claimed in claim 2 wherein the means for
counteracting the effect of said high impedance comprises a tuned
circuit, comprising a capacitor and a winding of the transformer,
for counteracting the effect of said high impedance on signals
received via the telephone line in the first state of the switching
means and having frequencies at about the resonance frequency of
the tuned circuit.
5. An interface circuit as claimed in claim 1, 3, or 4 wherein the
means for coupling signals from the telephone line to the receive
path comprises an amplifier, and the means for counteracting the
effect of said high impedance comprises means for increasing the
gain of the amplifier in the first state of the switching
means.
6. An interface circuit as claimed in claim 2 wherein the means for
coupling signals from the telephone line to the receive path
comprises an amplifier and the means for counteracting the effect
of said high impedance comprises means for increasing the gain of
the amplifier in the first state of the switching means.
7. Apparatus for receiving and transmitting signals via a
non-dedicated telephone line, said apparatus comprising an
interface circuit as claimed in claim 1, 4, or 6 and means
responsive to signals supplied via the telephone line to the
receive path for controlling the switching means.
8. An interface circuit for coupling a receive path and a transmit
path to a non-dedicated telephone line, said circuit
comprising:
a transformer having first and second windings;
first impedance means for coupling the first winding of the
transformer to the telephone line with a sufficiently high
impedance that normal use of the telephone line is undisturbed;
second impedance means coupled between the transmit path and the
second winding of the transformer;
amplifier means coupled between the second winding of the
transformer and the receive path; and
switching means selectively operable to decrease the impedances of
the first and second impedance means and to decrease the gain of
the amplifier means.
9. An interface circuit for coupling a receive path and a transmit
path to a non-dedicated telephone line, said circuit
comprising:
a transformer having first and second windings;
first impedance means for coupling the first winding of the
transformer to the telephone line with a sufficiently high
impedance that normal use of the telephone line is undisturbed;
second impedance means coupled between the transmit path and the
second winding of the transformer;
differential amplifier means having differential inputs coupled to
the second winding of the transformer and to the transmit path and
having an output coupled to the receive path; and
switching means selectively operable to decrease the impedances of
the first and second impedance means and to decrease the gain of
the differential amplifier means.
10. An interface circuit for coupling a receive path and a transmit
path to a non-dedicated telephone line, said circuit
comprising:
a transformer having first and second windings;
first impedance means for coupling the first winding of the
transformer to the telephone line with a sufficiently high
impedance that normal use of the telephone line is undisturbed;
second impedance means coupled between the transmit path and the
second winding of the transformer;
coupling means coupling the second winding of the transformer to
the receive path;
a capacitor connected to the second winding of the transformer and
forming a tuned circuit therewith, whereby signals having
frequencies at about the resonance frequency of the tuned circuit
and coupled to the first winding of the transformer via the first
impedance means are coupled via the coupling means to the receive
path; and
switching means selectively operable to decrease the impedances of
the first and second impedance means.
11. An interface circuit as claimed in claim 10 wherein the
coupling means comprises a differential amplifier having a
non-inverting input coupled to the second winding of the
transformer, an inverting input coupled to the transmit path, and
an output coupled to the receive path.
12. An interface circuit as claimed in claim 8, 9, or 10 wherein
said switching means is selectively operable to decrease the
impedance of the first impedance means to substantially zero and to
decrease the impedance of the second impedance means to
substantially match the characteristic impedance of the telephone
line.
13. Apparatus for receiving and transmitting signals via a
non-dedicated telephone line, said apparatus comprising an
interface circuit as claimed in claim 8, 9, or 10 and means
responsive to signals supplied via the telephone line to the
receive path for controlling the switching means.
14. Apparatus for connection to a non-dedicated telephone line,
said apparatus comprising an interface circuit as claimed in claim
1, 3, or 4 and signalling means connected to the receive and
transmit paths of the interface circuit, the signalling means being
responsive to a predetermined combination of signals supplied to
the receive path via the telephone line and the interface circuit
with the switching means of the interface circuit in said first
state to cause the switching means to adopt said second state.
Description
This invention relates to interface circuits for connection to
non-dedicated telephone lines, and to apparatus including such
interface circuits.
Telephone lines are increasingly being used for the transmission of
signals other than ordinary telephony signals. In order to effect
bidirectional transmission of such signals, it is known to couple a
modem to a telephone line. In the case of a non-dedicated telephone
line, i.e. a telephone line which is not used solely for
transmission of the modem signals, it has been necessary to couple
the modem to the telephone line for the transmission of the signals
only when required, following a predetermined call-up procedure.
Otherwise the connection of the modem would disturb telephony
signals ordinarily transmitted via the telephone line.
It is desirable, however, to eliminate the predetermined call-up
procedure and to provide a permanent coupling of the modem to the
telephone line, without loading the telephone line to a sufficient
extent that telephony signals are disturbed. It has been determined
that a permanently coupled modem should present a relatively high
impedance, for example at least 50 k.OMEGA., to the telephone line
in order to avoid disturbing telephony signals. The use of such a
high terminating impedance has resulted in an unacceptable signal
loss in coupling signals between the modem and the telephone
line.
In order to facilitate data collection, for example for remote
metering purposes, via a non-dedicated telephone line it is known
to connect a data transmitter to the telephone line, and to access
the data transmitter by means of ringing signals supplied via the
telephone line and initiated by a remote data collection center.
Such arrangements include a muting circuit for preventing ringing
of the subscriber's telephone when the data transmitter is
accessed. Such arrangements generally involve at least one of the
disadvantages that the muting circuit also mutes parts of ordinary
ringing signals; the ringing signals and/or the subscriber's
telephone must be modified; and the data transmitter may be
accessed in response to erroneous ringing signals arising from
ordinary telephone calls.
Accordingly, an object of this invention is to provide an interface
circuit for connection to a non-dedicated telephone line, which can
be permanently connected to the telephone line without disturbing
the normal use of the telephone line for the transmission of
telephony signals and without necessitating any modification of a
subscriber's telephone connected to the telephone line or of
ringing signals supplied thereto.
According to one aspect of this invention there is provided an
interface circuit for connection to a non-dedicated telephone line,
said circuit comprising: switching means having a first state and a
second state; means for presenting a high impedance to the
telephone line in the first state of the switching means, whereby
normal use of the telephone line is undisturbed; means for coupling
signals from a transmit path to the telephone line and from the
telephone line to a receive path in the second state of the
switching means; and means for counteracting the effect of said
high impedance on signals received via the telephone line in the
first state of the switching means, whereby said signals are
coupled to the receive path.
Preferably the means for coupling signals comprises a transformer
and the means for presenting a high impedance comprises a high
impedance via which a winding of the transformer is coupled to the
telephone line in the first state of the switching means, the high
impedance being by-passed in the second state of the switching
means.
The means for counteracting the effect of said high impedance can
comprise a tuned circuit, conveniently comprising a capacitor and a
winding of the transformer, for counteracting the effect of said
high impedance on signals received via the telephone line in the
first state of the switching means and having frequencies at about
the resonance frequency of the tuned circuit, and/or can comprise
means for increasing, in the first state of the switching means,
the gain of an amplifier which couples signals from the telephone
line to the receive path. The former arrangement is particularly
applicable to the receipt of frequency shift signals from the
telephone line, the tuned circuit resonance frequency being
selected in dependence upon the frequencies of the received
signals.
This invention also extends to an apparatus for connection to a
non-dedicated telephone line, said apparatus comprising an
interface circuit as recited above and signalling means connected
to the receive and transmit paths of the interface circuit, the
signalling means being responsive to a predetermined combination of
signals supplied to the receive path via the telephone and the
interface circuit with the switching means of the interface circuit
in said first state to cause the switching means to adopt said
second state.
According to another aspect the invention provides an interface
circuit for coupling a receive path and a transmit path to a
non-dedicated telephone line, said circuit comprising: a
transformer having first and second windings; first impedance means
for coupling the first winding of the transformer to the telephone
line with a sufficiently high impedance that normal use of the
telephone line is undisturbed; second impedance means coupled
between the transmit path and the second winding of the
transformer; amplifier means coupled between the second winding of
the transformer and the receive path; and switching means
selectively operable to decrease the impedances of the first and
second impedance means and to decrease the gain of the amplifier
means. The amplifier means is preferably a differential amplifier
means having differential inputs coupled to the second winding of
the transformer and to the transmit path and having an output
coupled to the receive path.
According to a further aspect the invention provides an interface
circuit for coupling a receive path and a transmit path to a
non-dedicated telephone line, said circuit comprising: a
transformer having first and second windings; first impedance means
for coupling the first winding of the transformer to the telephone
line with a sufficiently high impedance that normal use of the
telephone line is undisturbed; second impedance means coupled
between the transmit path and the second winding of the
transformer; coupling means coupling the second winding of the
transformer to the receive path; a capacitor connected to the
second winding of the transformer and forming a tuned circuit
therewith, whereby signals having frequencies at about the
resonance frequency of the tuned circuit and coupled to the first
winding of the transformer via the first impedance means are
coupled via the coupling means to the receive path; and switching
means selectively operable to decrease the impedances of the first
and second impedance means.
Preferably the switching means is selectively operable to decrease
the impedance of the first impedance means to substantially zero
and to decrease the impedance of the second impedance means to
substantially match the characteristic impedance of the telephone
line.
The invention also extends to apparatus for receiving and
transmitting signals via a non-dedicated telephone line, said
apparatus comprising an interface circuit as recited above and
means responsive to signals supplied via the telephone line to the
receive path for controlling the switching means.
The invention will be further understood from the following
description with reference to the accompanying drawing, in
which:
FIG. 1 illustrates apparatus including an interface circuit
according to an embodiment of the invention; and
FIG. 2 illustrates an interface circuit according to another
embodiment of the invention.
Referring to FIG. 1, there is shown therein an interface circuit 1
which serves to permanently couple a receive path 2 and a transmit
path 3 to the tip wire 4 and the ring wire 5 of a telephone line 6
which connects a subscriber's telephone 7 to a central office
(C.O., not illustrated).
The interface circuit 1 includes a surge protection device 8 which
is connected in series with a current limiting resistor 9 across
the line 6. Connected in series with one another and in parallel
with the device 8 are a coupling capacitor 10, the balanced
windings 11 of a line transformer 12, and a high impedance resistor
13. A normally open switch 14 is connected in parallel with the
resistor 13. Another winding 15 of the line transformer 12 is
connected via a coupling capacitor 16 to a junction point 17. The
junction point 17 is connected via a high impedance resistor 18,
and via a low impedance resistor 19 in series with a normally open
switch 20, to the transmit path 3. A differential amplifier 21 has
its non-inverting input connected to the junction point 17, its
inverting input connected via a resistor 22 to the transmit path 3,
and its output connected to the receive path 2. A feedback path
from the output to the inverting input of the amplifier 21 includes
series resistors 23 and 24. A normally open switch 25 is connected
in parallel with the resistor 23. As shown by a broken line, the
switches 14, 20, and 25 are constituted by contacts of a relay
26.
For example, various components in the interface circuit 1 may have
the following values:
resistor 9: 20.OMEGA.;
capacitors 10 & 16: 2 .mu.F;
resistors 13 & 23: 100 k.OMEGA.;
resistor 18: 200 k.OMEGA.;
resistor 19: 560.OMEGA.;
resistors 22 & 24: 12 k.OMEGA..
It is assumed here that signals to be transmitted via the interface
circuit 1 to and from the telephone line 6 are frequency shift
keyed (FSK) signals, for example at frequencies of 1070 and 1270 Hz
in the receive direction from the telephone line 6 to the receive
path 2 and at frequencies of 2025 and 2225 Hz in the transmit
direction from the transmit path 3 to the telephone line 6.
Accordingly, the receive path 2 is coupled via a bandpass receive
filter 27, having a passband including the receive frequencies, to
a modem, and the modem is coupled via a low pass transmit filter
28, having a passband including the transmit frequencies, to the
transmit path 3. The modem is coupled in known manner to a
universal asynchronous receiver-transmitter (UART) and thence to a
microprocessor. For simplicity the modem, UART, and microprocessor
are shown as a single block 29 in FIG. 1. The output of the receive
filter is also connected to a level detector 30 whose output
enables the UART for receiving signals, and the relay 26 is
controlled by the microprocessor, as described below.
In operation, the switch 14 is normally open so that a high
impedance of about 100 k.OMEGA. is presented to the telephone line
6. As a result of this high impedance the interface circuit 1 can
be permanently connected to the telephone line 6 without disturbing
telephone signals thereon. This high impedance is substantially
matched at the opposite side of the line transformer 12 by the high
impedance of the resistor 18, the switch 20 also being open. As the
switch 25 is open, the amplifier 21 provides a gain for received
signals equal to the ratio of the feedback resistance of the
resistors 23 and 24 to the resistance of the resistor 22. This gain
is selected to substantially compensate for the signal loss
suffered by received signals due to the high impedance resistor
13.
In order to establish communication via the telephone line 6, when
it is idle, and the interface circuit 1, a remote signal source
such as a data collection center is coupled to the telephone line 6
via the central office and supplies for example a constant mark
(1270 Hz) which is conducted via the interface circuit 1 and the
receive filter 27 to the level detector 30, which detects this
signal and supplies an enabling signal to the UART. The remote
signal source then supplies an FSK identity code to the telephone
line 6, which code is similarly conducted via the interface circuit
1 and the receive filter 27 to the block 29, where it is compared
by the microprocessor with a corresponding locally stored identity
code. In the event that the compared codes are the same, the
microprocessor controls the relay 26 to close the switches 14, 20,
and 25 and triggers transmission of a ready signal, such as a
constant mark (2225 Hz), from the block 29 via the transmit filter
28, the interface circuit 1, and the telephone line 6 to the remote
signal source. Communication between the block 29 and the remote
signal source can then take place, via the interface circuit 1 as
described below, using FSK signalling in known manner. At the end
of such communication, or in the event of an interruption of the
communication due to the presence of telephony signals on the
telephone line 1, the microprocessor in the block 29 controls the
relay 26 to re-open the switches 14, 20, and 25, thereby returning
the interface circuit to its original state.
The closure of the switches 14, 20, and 25 under the control of the
relay 26 as described above results in the high impedance resistor
13 being bypassed by the switch 14, removing the signal losses
associated therewith. At the same time the resistor 23 is bypassed
by the switch 25, so that the gain of the amplifier 21 is reduced
(e.g. to unity), the gain of this amplifier no longer being
required because the signal losses are removed. Furthermore, the
closing of the switch 20 reduces the impedance between the transmit
path 3 and the junction point 17 to a value which substantially
matches the characteristic impedance (e.g. 600.OMEGA.) of the
telephone line 6 and which permits transmit signals to be supplied
from the transmit path 3 to the telephone line 6.
FIG. 2 illustrates a modified form of interface circuit, the same
references being used to denote the same components as in FIG. 1.
The circuit shown in FIG. 2 differs from that of FIG. 1 only in
that the switch 25 and the resistor 23 are dispensed with, so that
only the resistor 24 appears in the feedback path of the amplifier
21 which consequently is a constant (e.g. unity) gain amplifier,
and a capacitor 31 is connected in parallel with the winding 15 of
the line transformer 12 to form a tuned circuit therewith. The
capacitance of the capacitor 31 is selected to be such that the
resonance frequency of the tuned circuit is similar to the
frequencies of the FSK signals to be received. For example, the
resonance frequency f of the tuned circuit is such that f.sub.2
=fm, fs, where fm and fs are the frequencies of the mark and space
receive signals respectively. Thus for fm=1270 Hz and fs=1070 Hz as
described above, f=1166 Hz. For example the capacitor may have a
capacitance of about 0.01 .mu.F in order to provide such a
resonance frequency.
The resonance of the tuned circuit at about the receive frequencies
significantly reduces the signal loss for such signals when the
switches 14 and 20 are open, thus counteracting the effect of the
high impedance resistor 13 for the receive frequencies. The tuned
circuit also acts as a bandpass filter when the switches are open,
thereby serving to reduce interference signals. As the effect of
the high impedance resistor 13 is counteracted by the tuned
circuit, the additional gain for received signals provided by the
amplifier 21 in conjunction with the open switch 25 and the
resistor 23 in FIG. 1 is not required, so that the switch 25 and
the resistor 23 are omitted with a consequent simplification of the
circuit and the switching requirements. However, these components
may also be provided as well as the tuned circuit, if additional
gain is required for received signals when the switches 14 and 20
are open.
When the switches 14 and 20 in the circuit of FIG. 2 are closed, in
the same manner as described above with reference to FIG. 1, the
circuit of FIG. 2 operates in the same manner as that of FIG. 1. In
this situation the reactance of the capacitor 31 is very much
larger than the impedance presented by the resistor 19 which is
connected via the switch 20, so that the tuned circuit has
substantially no effect on the transmitted and received
signals.
Whilst the interface circuit is not limited to any particular
application, it is observed that it may particularly be used in a
subscription television system for providing a connection between a
subscriber's television unscrambling equipment and viewed-program
memory and his telephone line, whereby viewed-program information
stored in the memory may be read from time to time via the
telephone line and new decoding information may be supplied to the
unscrambling equipment via the telephone line, without disturbing
the subscriber or his telephone service. As should be appreciated
from the above description, the use of the interface circuit does
not involve any subscriber action and does not result in ringing of
the subscriber's telephone. Furthermore, the interface circuit does
not require any modification of the subscriber's telephone or its
operation, or of the ringing signals which are supplied thereto.
The interface circuit may be used similarly for remote access to
utility metering equipment, for office communications systems, and
so on.
Although the interface circuit has been described in relation to
the transmission and reception of FSK signals, it is of course not
limited thereto and may be used for the reception and transmission
of signals of arbitrary frequencies and in accordance with various
signalling schemes. However, in the case of the interface circuit
of FIG. 2 it is of course initially necessary to supply a signal at
about the resonance frequency of the tuned circuit in order to
effect the desired closing of the switches 14 and 20.
The invention is similarly not limited to the particular switching
and coupling arrangements described, and numerous modifications,
adaptations, and variations may be made therein and elsewhere in
the described embodiments without departing from the scope of the
invention as defined by the claims.
* * * * *